We describe the fabrication of high-stress Si_3N_4 nanobeam resonators with high aspect ratios exceeding lengths of 3.5mm. The lowest order out-of-plane modes of these nanobeams have quality factors of Q\geq 10^6 with fundamental mode frequencies lying in the range of 80-500kHz. The beams are fabricated from high-stress, 20-50nm-thick films of Si_3N_4 deposited via LPCVD on standard silicon wafers. The beams are patterned via electron beam lithography and deep reactive ion etching. The underlying silicon is etched in KOH and the released structures are dried in a critical point dryer.
 A. H. Ghadimi, S. A. Fedorov, N. J. Engelsen, M. J. Bereyhi, R. Schilling, D. J. Wilson, and T. J. Kippenberg, Science 360, 764 (2018).  M. J. Bereyhi, A. Beccari, S. A. Fedorov, A. H. Ghadimi, R. Schilling, D. J. Wilson, N. J. Engelsen, and T. J. Kippenberg, Nano Lett. 19, 2329 (2019).  S. A. Fedorov, A. Beccari, N. J. Engelsen, and T. J. Kippenberg, Phys. Rev. Lett. 124, 025502 (2020).  S. A. Fedorov, N. J. Engelsen, A. H. Ghadimi, M. J. Bereyhi, R. Schilling, D. J. Wilson, and T. J. Kippenberg, arXiv:1807.07086.
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Interesting work. Is it extendable to other materials?
Mohammad J. Bereyhi
Not exactly. Our KOH undercut process is optimized for the highest selectivity between silicon and silicon nitride and depending on a change in the material it can lead to different results.